We intend to continue studying the mechanisms that render the nicotinic synapse a minute but highly sophisticated electrochemical machine, specialized to function on a time scale of milliseconds and a distance scale of micrometers. Electrophysiological and photochemical methods will be applied to electroplaques from Electroporus and Torpedo and to muscle fibers from frogs and fish. We shall investigate the agonist-receptor interaction using photoisomerizable agonist molecules. The experiments will probe (a) the role of membrane surface charge in accelerating the agonist-receptor interaction, (b) the number of bound agonist molecules directly linked to channel activation, (c) gating charge movements associated with agonist-receptor interaction, and (d) the mechanism of desensitization. We shall investigate the interaction between antagonists and the receptor by employing photochemically induced increases and decreases of antagonist concentration. We shall study the molecular nature of the rate-limiting steps leading to channel activation. On a millisecond time scale, binding of fluorescent drugs (agonists, antagonists, and open-channel blockers) will be measured and compared with the simultaneously measured membrane conductance. We shall develop methods for internally perfusing Electrophorus electroplaques. We shall then examine the role of internal drugs and ions in channel gating, ion movement through the channel, and desensitization. We shall conduct quantitative electrophysiological investigations on Torpedo electroplaques, for comparison with published binding and flux data on receptor-rich membrane fragments and on reconstituted membranes.